The measurement of carriage height in forklift vehicles is a critical parameter affecting speed and stability. In the aforementioned U.S. Pat. No. 4,942,529, it is disclosed that the height of the carriage of a forklift truck can be determined by measuring the hydraulic flow or displacement necessary to lift and lower the mast supporting the carriage. The fluid flow measurement is converted into electrical signals that are used to determine the height of the carriage of the lift truck from a home position. While the disclosure provides a viable means to accomplish the objectives outlined therein, the measurement of fluid flow comprises many complexities that were not addressed by the patent. These complexities affect the precision of the measurement and could in an extreme situation lead to an erroneous result.
The present invention has as one of its objectives to provide a method and apparatus for obtaining a more precise hydraulic flow measurement, and hence for determining the proper height of the carriage of the forklift vehicle.
One of the deficiencies of the fluid measurement was the failure to account for the variations introduced by reason of viscosity changes in the hydraulic fluid. Viscosity is a function primarily of temperature of the fluid.
It was also observed that the flow sensor output frequency will differ, depending on the rate and direction of the fluid flow. This is due to the mechanical construction of the sensor itself. In other words, the sensor is not truly bidirectional in its characteristics with regard to fluid flow.
The present invention seeks to compensate for changes of viscosity based on fluid temperature and for flow rate based on flow sensor frequency output.
The present invention also seeks to correct for the non-symmetrical operating characteristics of the fluid sensor itself.
It is an object of the present invention to determine absolute carriage height accurately by means of a flow sensor.